Brendan Larder reported
data from a test of over 125 clinical isolates with varying degrees of
cross-resistance to indinavir, ritonavir, nelfinavir, and saquinavir in a
recombinant phenotypic assay. These isolates ranged from having resistance to a
single PI with a limited number of PI mutations to those highly co-resistant and
containing 6 to 8 protease mutations.
The sample selection was
from previously characterized PI resistant clinical isolates in Virco’s
repository, representing a broad spectrum of PI susceptibility:
Highly
PI cross-resistant (to 3 or 4 PIs) n=107
Single resistance
to ritonavir, nelfinavir or saquinavir. They could not find single
resistance to indinavir. Any indinavir resistant mutant was also
cross-resistant to other PIs n=28
All viruses were
sequenced to confirm mutation pattern before phenotypic analysis. They used the
VircoGen method which uses ABI genotypic sequencing and the Virco Antivirogram
phenotypic assay.
Results.
96/107 (90%) isolates showed susceptibilty to TPV. Eight (7.5%) showed
intermediate levels of resistance to TPV. Three of 107 (3%) showed resistance
with changes in the IC50 >10-fold. Of the 107 viruses cross-resistant to PIs
the resistance to the other PIs ranged from 40 to 90 fold. TPV resistance was
2-fold. At least 85 isolates had >10-fold resistance to all four PIs. Among
the highly cross-resistant isolates the more prevalent mutations were at 90, 71,
10, 48, 54, 82A, 84. Larder reported all of the samples resistant to single PIs
(RTV, NFV, or SQV) remained fully TPV susceptible or were hypersensitive.
Viruses resistant only
to NFV (n=10) or RTV (n=13) had no resistance to TPV, but the 5 saquinavir
resistant viruses showed a 2.5 fold hypersenstivity to TPV. Larder reported 21
isolates with high-level resistance to the 4 PIs had 3.3 fold hypersensity to
TPV. The ritonavir-only resistant viruses displayed mutations at 10, 46, 54, 71,
77, 82A, 82T, 84, and 90. The nelfinavir-only resistant viruses showed
predominantly the D30N, which is the hallmark nelfinavir mutation. Almost 100%
of these viruses had a D30N and 88 mutation. Also observed were mutations at 10,
36, and 71. The saquinavir mutations observed were a predominance of 48
and 90 and also 10, 36, 46, 54, 71, and 77.